How to Make It
Heat the olive oil in a very large saucepan. Add the leeks, celery and potato and cook over moderately low heat, stirring often, until the vegetables are slightly softened, about 10 minutes. Add the chopped lettuce by large handfuls, stirring each batch until wilted before adding the next. Add 2 cups of the vegetable stock and season with salt and pepper. Reduce the heat to moderately low. Cover and simmer the soup, stirring often, until the vegetables are very soft, about 20 minutes. Add the remaining 2 cups of stock, cover and cook for 20 minutes longer.
Working in batches, carefully puree the hot soup in a blender. Pour the soup into a clean saucepan and season with salt and pepper.
In a medium bowl, prepare an ice water bath. Set aside 8 small watercress sprigs. In a medium saucepan of boiling salted water, blanch the remaining watercress and the basil leaves for 5 seconds. Drain and plunge into the ice water bath to cool. Drain again and squeeze out any excess water, then coarsely chop. Transfer the chopped watercress and basil to a blender. Add the water, chives, extra-virgin olive oil and marjoram and blend until emulsified. Transfer the puree to a bowl and season with salt and pepper.
Gently reheat the soup and ladle into warmed shallow bowls. Drizzle some Tuscan olive oil and a rounded tablespoon of the watercress puree over the soup. Garnish with the reserved watercress sprigs and serve.